Computer systems typically include one or more computers, referred to as hosts. Where multiple computers are used, the computers are usually interconnected by a network that allows them to share data. Typically, such networks also include one or more data storage devices to provide additional data storage capacity for the networked computers. A common data storage device is a disk array, also sometimes referred to as a Redundant Array of Independent (or Inexpensive) Disks (RAID). A disk array or RAID is two or more hard drives or similar disks that provide data storage for connected hosts.
Redundant Arrays of Independent (or Inexpensive) Disks are intended to provide storage with better performance and reliability than individual disks. In the pursuit of better performance or better reliability, numerous RAID types have been devised. Each of these RAID types has different reliability and performance characteristics.
For example, one RAID Type-is called Type-1. With a Type-1 RAID, data that is written to one disk is simply mirrored to another, second disk. Thus, data stored in a Type-1 RAID is very reliable because all the data is stored twice and is, therefore, automatically backed up against inadvertent error or corruption.
In a Type-1 disk array with N disks, there are N/2 disks worth of data storage space, and up to N/2 different input or output operations (I/O) can be accommodated at any given time (one I/O per 2 disks in the array). Thus, a Type-1 RAID sacrifices storage capacity to a certain extent in favor of higher reliability and performance.
Another RAID Type-is called Type-5. In Type-5 RAID, one disk's worth of parity information is calculated from the other disks in the array. Parity refers to an integer's property of being odd or even. Parity checking is used to detect and correct errors in binary-coded data, including data stored on a disk array, such as a RAID.
Therefore, in a Type-5 disk array with N disks, there are N-1 disks worth of data storage space. However, when a write command is entered into a Type-5 array, the array must first read from two disks as part of the parity calculation and then write to two disks afterward. As a result, a Type-5 array can only handle about half as many input/output commands in a given amount of time as a Type-1 array. Thus, a Type-5 array has a larger storage capacity for a given number of disks at the sacrifice of some input/output speed.
Given this general information, it becomes apparent that each RAID Type- has its own strengths and weaknesses. A Type-5 array can fit more user data onto a given number of disks than can a Type-1 array. However, a Type-5 array processes input/output commands at a rate only about half that of a Type-1 array.
The strengths and weaknesses of individual RAID types have given rise to the hierarchical storage array. In a hierarchical storage array, data is moved from RAID Type-to RAID Type-based on certain criteria in order to try to obtain more of the strengths and less of the weaknesses than each of the individual RAID types offer by themselves.
For example, consider a hierarchical storage array that could keep a small subset of its data in Type-1 RAID system and the rest of its data in a Type-5 RAID system. If the data in the system is manipulated to ensure that all of the I/Os coming into the system are only going to Type-1 storage, then the array would have the I/O performance of a Type-1 RAID. However, the array has more storage capacity than a typical Type-1 RAID with the same number of disks because data that is not used in an active I/O is transferred to, and kept in, the higher capacity Type-5 portion of the system. The key to this advantage is the data manipulation. Data must be moved from Type-1 storage to Type-5 storage when it is no longer being accessed, and data that is being accessed regularly must be moved to Type-1 storage from Type-5 so that all future accesses will be at the higher I/O rate that a Type-1 RAID provides.
In the current art, the way that data has its RAID type changed is by physically relocating it from one location to another. In other words, each location in the array is designated as being part of the Type-1 system or part of the Type-5 system. A single disk may have both Type-1 and Type-5 locations, but each location on each disk of the array is dedicated to either the Type-1 or the Type-5 system.
When data is to be exchanged from the Type-1 to the Type-5 system, or vice versa, the data is physically moved from a location of the first type to another location within the array of the second type. This strategy works well unless the array is at or near data storage capacity. If the array is full or very close to full, there may be no free location of the second storage type to which data of a first type can be moved. When this occurs, data manipulation must stop and the array can no longer function in a hierarchical fashion providing both Type-1 and Type-5 advantages.